**Core Concept:** The Sphincter of Oddi (SoO) is a muscular ring that controls the flow of bile and pancreatic juice from their respective ducts into the common bile duct and duodenum. It is located at the ampulla of Vater, where the common bile duct, cystic duct, and pancreatic duct meet.

**Why the Correct Answer is Right:** The correct answer is **D** because it explains that the Sphincter of Oddi consists of smooth muscle layers arranged in an "I" shape, which allows for coordinated and controlled release of bile and pancreatic juice. This arrangement ensures that the ducts do not become obstructed or over-contracted, preventing complications such as cholangitis and pancreatitis.

**Why Each Wrong Option is Incorrect:**

A. **Incorrect**: The Sphincter of Oddi is not composed of bile duct and pancreatic duct walls, but rather, it consists of smooth muscle layers from the ampulla of Vater.

B. **Incorrect**: While the ampulla of Vater is the location of the Sphincter of Oddi, this answer does not specify its composition.

C. **Incorrect**: This answer refers to the ampulla of Vater, which is the point where the bile and pancreatic ducts meet, but does not mention the smooth muscle composition of the Sphincter of Oddi.

**Clinical Pearl:** Understanding the structure and function of the Sphincter of Oddi is crucial for preventing complications related to bile and pancreatic duct obstructions. Adequate knowledge of this structure is essential for clinical practice, especially in managing patients with gallstones and pancreatitis.

**Correct Answer:** D. Smooth muscle of the ampulla of Vater